The invention relates to precise control of gases at high pressure.
There are various situations in which gas pressure must be precisely controlled, for example in low gas pressure measurement, control, or calibration instruments and in high gas pressure measurement, control, or calibration instruments. The present state of the art limits the precision with which gas pressure can be controlled at pressures above about 1000 to 2500 pounds per square inch to about .+-.0.05 percent. Generally, the more precise currently available high gas pressure devices are, the slower and less reliable they are. Below approximately 2500 pounds per square inch gas pressure, some available devices are capable of precision of better than .+-.0.01 percent, but above 2500 pounds per square inch gas pressure presently available devices have precision limited to about .+-.0.05 percent.
The present assignee markets a computerized pressure controller system that is capable of setting and stabilizing gas pressure to within .+-.0.001 percent or better, if the gas pressures are less than approximately 1000 pounds per square inch, by differentially opening and closing both an inlet solenoid valve and an exhaust solenoid valve connected to common pressure chamber. That computerized system can repeatedly measure the pressure in a pressure chamber and operate on the measured data by means of an algorithm to differentially open and close both the inlet solenoid valve and the outlet solenoid valve so as to produce very minute controlled increases or decreases in pressure in the pressure chamber. In some prior systems, variable orifice valves are controlled to produce a desired pressure in the pressure chamber (and in a control volume). In such devices which use solenoid valves or variable orifice valves or screw presses, mechanical hysteresis associated with the valves or seals of screw presses causes loss of precision in maintaining the controlled pressure to within desired limits. Such mechanical hysteresis prevents mechanical valves or screw presses from operating ideally, and increases with time and wear of the valves. It is very desirable to be able to avoid the effects of mechanical hysteresis in gas control valves variable orifice valves, or screw presses.
Unfortunately, the above described technique using mechanical valves is impractical at pressures substantially higher than about 1000 to 2000 pounds per square inch, because as the gas pressure increases the mechanical solenoid valves require proportionately higher amounts of physical force on the valve elements, and the amounts of power required to produce such forces also increase as the pressure of gas increases. Because of the higher physical forces on the valve elements, there is rapid wear of the seals and the seat of the valves, resulting in gas leakage that deteriorates performance of the system. Also, thermal problems become much more severe at the higher power levels, and power supplies become much more expensive; the thermal problems referred to occur because coils in the solenoid valves become hot, and that causes gas in the system to be heated in an uncontrolled manner, producing undesired and uncontrolled gas pressure changes. The only commercially available systems that operate at gas pressures above roughly 1000 to 2000 pounds per square inch are not as reliable as is often desired, and reliability decreases as the resolution is increased. Consequently, mechanical valves frequently must be rebuilt at considerable cost and inconvenience. System performance deteriorates gradually as the above-mentioned mechanical hysteresis and limited resolution problems become worse and as the valves begin to leak.
Most prior high gas pressure control systems that attempt to achieve high resolution, especially systems using variable orifice valves, waste large amounts of gas.
Those skilled in the art will recognize that it is very important that no pressure overshoot occur in a high gas pressure control system used for calibrating pressure measurement, control, or calibration devices, because pressure overshoot prevents accurate characterization of the hysteresis of such devices. It should be appreciated that it is very difficult to achieve speed in measurement or calibration of such devices without "overshooting" of the control pressure, because of the above-mentioned limitations in overall resolution of prior devices due to the resolution limitations of the mechanical valve components. Similarly, if the controlled high gas pressure being generated is used to control a manufacturing process, overshooting of the pressure above the target value often is not acceptable.
Avoiding pressure overshooting while obtaining fast attainment of the target pressure presents a formidable design challenge that has not been successfully met in the art. There is an unmet need for an economical and reliable system for more rapidly and precisely controlling highly pressurized gas than has previously been achieved.